This doesn't seem a recent regression, the underlying issue is that we don't account for
the Extensions content scripts (and user scripts) as other principals that may be able
to intercept a drop event and then try to access the dataTransfer.files property before
the webpage does.
As the big inline comment inside DataTransferItemList::Files does explain, keeping a copy
of the FileList when accessed from a webpage principal is necessary for spec compliance,
while we don't cache it when it is being accessed by system principal code.
(see https://searchfox.org/mozilla-central/rev/6cbe34b441f7c7c29cd1e5f0e19c7000142f1423/dom/events/DataTransferItemList.cpp#200-242)
The changes in this patch are preventing us from caching the file list also when accessed
by expanded principals (which are never going to be used for any web page, but they are used
by both Extensions content scripts and Extensions user scripts), along with adding a new.
regression test to prevent it from regressing without being noticed.
Without this patch, when an extension content script or user script does intercept the
drop event and access the dataTransfer.files property, we cache the FileList created
for the ExpandedPrincipal associated to the Extension script and after that if the page
try to access it we do return null because the webpage principal doesn't subsume the
expanded principal (which is also why the issue isn't triggered if the webpage does
access it first). In debug build we crash because we hit the assertion in
DataTransferItemList::Files right before the earlier nullptr return we hit on release.
Differential Revision: https://phabricator.services.mozilla.com/D113555
An explanation of the Firefox Source Code Directory Structure and links to
project pages with documentation can be found at:
https://firefox-source-docs.mozilla.org/contributing/directory_structure.html
For information on how to build Firefox from the source code and create the patch see:
https://firefox-source-docs.mozilla.org/contributing/contribution_quickref.html
If you have a question about developing Firefox, and can't find the solution
on https://firefox-source-docs.mozilla.org/, you can try asking your question on Matrix at chat.mozilla.org in `Introduction` (https://chat.mozilla.org/#/room/#introduction:mozilla.org) channel.
Nightly development builds can be downloaded from:
https://archive.mozilla.org/pub/firefox/nightly/latest-mozilla-central/
- or -
https://www.mozilla.org/firefox/channel/desktop/#nightly
Keep in mind that nightly builds, which are used by Firefox developers for
testing, may be buggy.
166d68dc58